Cracks have appeared in the new French government after a Socialist minister made a gaffe about the future of nuclear power and tensions mounted over the thorny issue of parliamentary ratification of the European budget treaty.

The Socialist president, François Hollande, and his prime minister, Jean-Marc Ayrault, have seen their popularity ratings fall over the summer as the French economic crisis deepens. Already struggling with a difficult return to work after the holiday season, the government has now been shaken by the row over nuclear energy.

The Green party, which has two ministers in the Socialist-led government, was taken aback after the minister for industrial recovery, Arnaud Montebourg, described nuclear power as an "industry of the future", seeming to cast doubt on a commitment to shut power stations and reduce France's devotion to atomic energy. France is the most nuclear-dependent country in the world, with 75% of its energy coming from nuclear. In a deal with the Greens before this year's parliamentary and presidential elections, Socialists promised to reduce the share of nuclear in French electricity production to 50% by 2025, shutting 24 nuclear reactors. But so far, only one of France's 59 nuclear reactors, at Fessenheim in eastern France, is due to be decommissioned.

Montebourg called nuclear energy a "tremendous asset" with a key future role, saying: "We need energy that is not too expensive." He was backed by the interior minister, Manuel Valls, who said nuclear was undeniably a part of the future of French industry.

The Green MP Noël Mamère condemned the comments as a provocation. Another Green MP, Denis Baupin, said the government was "divorced from reality". The prime minister tried to play down the row, and Montebourg's views were presented as personal opinions. But the timing was unfortunate, two weeks before the government's big environmental conference, designed to convince a sceptical environmental lobby that Hollande is serious about green issues.

Meanwhile, the Socialists are trying to contain internal divisions over the ratification of the new European budget treaty. Relieved that the French constitution will not need to be modified to accommodate the treaty, the government must still submit the text to a parliament vote in October.

But some in the Greens and on the left wing of the Socialist party have misgivings about losing sovereignty to Brussels and imposing austerity rules which they say would be socially and economically damaging to France.

Jean-Luc Mélenchon's leftist Front de Gauche and Eva Joly, the former Green presidential candidate, have called for a referendum. The debate around the treaty ratification is already reopening political wounds from the sparring about the 1992 Maastricht treaty and France's 2005 no vote against the EU constitution.

The prime minister has taken a hard line, telling Socialist MPs they should fall in line with the government amid "risk of a European crisis".

Meanwhile, in its first key move after the summer break, the government on Tuesday temporarily cut the price of petrol and diesel by up to six cents a litre to help French households. The cost will be shared between oil companies and the state, which will lose €300m in tax revenues. The move stopped short of Hollande's campaign promise of a freeze on fuel prices, which had been seen as too complicated and a legal minefield.